1. Field of the Invention
The present invention relates to an electric power steering apparatus.
2. Description of Related Arts
There is provided one type of electric power steering apparatus for automobiles that amplifies an output of a motor for steering assist by a speed reduction mechanism comprising a pinion and a wheel meshed with the pinion.
What is proposed as this type of electric power steering apparatus has been an apparatus in which the whole or the surface of a tooth section of at least one of a pinion and a wheel is formed of synthetic resin in order to reduce vibration and noise caused by the mesh of the pinion and the wheel (see Japanese unexamined patent application 2002-308123, for example).
Furthermore, an electric power steering apparatus so adapted as to coat parts of the teeth of at least one of a pinion and a wheel with synthetic resin has been proposed (see Japanese unexamined patent application 07-232653 (1995), for example).
An example of electric power steering apparatuses is one of a so-called rack assist type in which an output of a motor is transmitted to a steering shaft (a rack shaft) through a motion conversion mechanism such as a ball screw mechanism in addition to the above-mentioned speed reduction mechanism.
In such an electric power steering apparatus of a rack assist type, it is important in reducing noise produced in a cabin of an automobile (a passenger compartment) to reduce noise produced by a speed reduction mechanism. Further, significantly high torque is inputted from a motor to the speed reduction mechanism. Therefore, the strength of the speed reduction mechanism must be sufficiently ensured.
When the whole of the tooth section is formed of synthetic resin, however, it may be impossible to sufficiently ensure the strength of the tooth section. On the other hand, when only the surface of the tooth section is formed of synthetic resin, it may be impossible to sufficiently absorb shock due to the mesh of both the pinion and the wheel and therefore, to sufficiently reduce noise.
The present invention has been made under such a background, and has an object to provide an electric power steering apparatus that can reduce noise and can ensure sufficient strength.